Assessing potential invasiveness of woody horticultural plant species using seedling growth rate traits

1. The ornamental plant trade, forestry, and agriculture have been responsible for the initial introduction of over 60% of invasive alien plant species. Screening tools to test potentially new horticultural species should help curtail the continued introduction of new invaders. 2. Using two methods for analysing phylogenetically independent contrasts (PICs) of known invasive and non-invasive, exotic woody horticultural species, we tested the potential of relative growth rate (RGR) and related traits including net assimilation rate (NAR), leaf area ratio (LAR), and specific leaf area (SLA) as predictors of invasiveness. These 29 PICs include 65 species and broadly cover angiosperms. 3. Without accounting for phylogeny, no significant differences were found in seedling RGR or related traits between invasive and non-invasive woody species. Using PICs, invasive species' RGRs were significantly higher. RGR was considerably more significant using our extensive dataset than in previous smaller studies, while SLA and LAR remained marginally significant. NAR was significantly higher for invasives for the 10-20 days interval. 4. Analysis of this broad data set confirms that RGR is significantly higher for invasive woody species than their non-invasive counterparts, and may serve as a useful biological predictor of invasiveness for woody angiosperms. This expanded study shows that plant species use different physiological and biomass allocation patterns to achieve higher RGR; therefore individual components of RGR, such as SLA, do not consistently predict potential invasiveness of species. 5. Synthesis and applications. Comparative seedling RGR studies show that this measure has potential as a screening tool for new exotic plant species. Unfortunately, more easily measurable components of RGR do not consistently predict invasiveness, as previously thought. Using seedling RGR analysis as an invasive species' screening tool requires growing a species proposed for introduction with related invasive and non-invasive species. If the tested species' RGR is higher or not significantly different from its known invasive counterpart, it should be considered highly likely to become invasive, and excluded from further consideration as a potential horticultural species. Seedling RGR could potentially produce a useful, straightforward screening tool when phylogenetically related species or cultivars are available.